The present invention related to a non-scanning type optical reading apparatus for reading patterns such as a bar code, an image of which is formed on a photosensor.
Conventional optical reading apparatus of the type contemplated by the present invention illuminate a pattern such as a bar code with a light source including LEDs for example, so that the reflected light from the pattern is projected on a light-receiving element such as a CCD line sensor by means of an imaging lens to read the information of the pattern from the output thereof.
In the conventional optical reading apparatus, the reflected light from the uniformly illuminated pattern is imaged, so the cos.sup.4 power law of the imaging lens causes light attenuation, whereby the line sensor receives less light in its periphery than in the central area. If the image on the line sensor is not uniform in the distribution of light quantity, it becomes difficult to set a threshold level in the circuit for processing the output of the line sensor. As a result, the information of the pattern can potentially be read incorrectly or the information thereof cannot be read on account of a setting error.
Apparatus such as a copying machine and a facsimile machine use a shading plate for correcting the light quantity. Compared with copying machines and facsimile machine, bar code readers use less bright lenses having large F numbers. Since such dark lenses transmit only a bundle of rays having a small diameter, the range of errors that are permissible for the size and position of the shading plate is relatively narrow. Hence, in case of using shading plates, it takes time to manufacture and install them, making the completed optical system quite unsuitable for large-scale production.